Stabilization of alkylene glycols



. Patented May 1 3 2,441,193 STABILIZATION OF GLYCOLS Edgar C. Brltton and Mich, amignors to PW, ware Arthur R. Sexton, Midland,

The Dow Chemical Coni- Midland, Mich, a corporation of Dela- No Drawing. Application August 6, 1945, Serial No. 609,317

This invention relates to stabilizing lower alkylene glycols against thermal decomposition.

In certain types of air craft and automotive engines, the liquid coolant, instead of running at the conventional temperature of 100 C. or less, commonly circulates at 150 C. in a closed system, and may occasionally reach 200 C. or higher, perhaps for an extended period. The problem of selecting a coolant which .will operate satisfactorily at these temperatures and yet will not freeze even at extreme winter temperatures is-a diflicult one. Ethylene glycol, undiluted or together with a lesser proportion of water, is most frequently chosen as the coolant. but it has the undesirable property of decomposing slowly at the higher temperatures mentioned to form organic acids, carbon, and gummy tars. These acids, when formed, tend to corrode the metallic parts of the circulating system, and the carbon and tar ultimately foul the heat-transfer surfaces and throttle circulation. In consequence, with ethylene glycol coolants, periodic overhauling of the circulatingsystem is necessary, sometimes at short intervals.

In an efiort to provide a more satisfactory liquid coolant for operation at high temperatures, it has now been found that the lower alkylene glycols, such as ethylene glycol, may be stabilized against thermal decomposition of the type mentioned by incorporating therein a small proportion of 1,4-piperazinediethanol. This compound almost completely restrains thermal decomposition of the glycols, both in the undiluted state and in mixtures thereof with the same or a lesser weight of water, at temperatures up to 200 C., and very markedly retards decomposition even when the glycols are maintained liquid under pressure at temperatures as high as 300 C.

Glycol compositions stabilized in accordance with the invention not only are useful as coolants for internal combustion engines but are satisfactory as liquid heat-transfer agents in general at temperatures within the range mentioned. They are characterized by extremely low freezing points, good heat-transfer characteristics, and moderate viscosities, as well as thermal stability. The compositions may also be used as fluid pressureand power-transmitting media.

The lower alkylene glycols to which the invention is applicable contain from two through five carbon atoms per molecule. Of these, ethylene glycol. and to a lesser extent industrial mixtures of propylene glycols and.of butylene gly- I Claims. (Cl. 252-77) cols, are of present commercial interest. Such glycols may be employed according to the invention in the undiluted state or mixed with an equal or lesser weight of water. With larger proportions or water, the boiling points of the solutions are somewhat low for a high temperature fluid, and thermal decomposition is rarely a serious problem.

The 1,4-piperazinediethanol is soluble in the undiluted glycols and their aqueous mixtures to a limited extent and may be incorporated in them simply by stirring in the crystals of the stabilizer. More conveniently, it may be made up as a strong solution in a mutual solvent, such as alcohol, and the solution stirred into the glycol composition. A small proportion of the 1,4- piperazinediethanol is all that is required to effect stabilization, 0.2 to 5 per cent by weight of glycol present being preferred.

The 1,4-piperazinediethanol, in addition to preventing carbon and tar formation, minimizes corrosion of metals by heated glycols and glycolwater mixtures by virtue of the fact that it almost completely restrains development of acidic substances by decomposition of the glycols. It also functions to a moderate degree as an inhibitor in the conventional sense of retarding the corrosion of metals .by other agents, such as air, which may be dissolved in the glycol solution. However, when extremely corrosive conditions are to be encountered, it is preferable to rely on the 1,4-piperazinediethanol as a thermal stabilizer only and to introduce in addition a small proportion of a conventional corrosion inhibitor. Since many such inhibitors are themselves unstable or volatile at temperatures of 200 to 300 C., it is important to select an inhibitor which is not afiected adversely by heating. Sodium silicate, usually from 0.5 to 5 per cent by weight of the glycol, is a preferred choice.

While the heat-transfer media of the invention ordinarily consist of the glycol and the stabilizer, with or without water, minor proportions of other ingredients such as anti-foamin agents, leak-stopping compounds, and dyes, as well as the conventional inhibitors already mentioned, may be added as desired.

The following examples will serve to illustrate the invention, but are not to be construed as limiting its scope. In the examples, atesting apparatus was employed consisting of an electrically heated closed iron chamber of about 400 cc. capacity and a circulating system for withdrawing the liquid being tested from a reservoir, forcing it under a pressure of to a number of rust particles, 7 odor, and had become acidic, the pH being 5.8.

v invention, the

, DH beln8 8.0. ,There the test specimens were:

not be determined.

, inch into one end of the heated chamber, removing it from the other end of the chamber through a relief valve, cooling it, and returning itto the reservoir- The ,rate of circulation was cont d hour. Polished test specimens of aluminum. iron, brass, and

3 pounds per square copper were placed in the heat-J. t

ed chamber throughout each run. ,The purposes 7 or the metal specimens were to have present in the system the metals normally usedin liquidcooled engines, corrosion of the metals.

Example. 1.Aqueous mixture at 200 C.

In a blank run. the system was filled'i' of a mixture of '70 parts by weight .012

2565 cc. ethylene glycol and 30 parts of water, containing nostabilizer; the pH 01' this mixture was 7.3.

The heatedchamber wasmaintainedat 200 C., circulation being continued for 100 hours. The 7 metal test specimens present in the chamber were weighed at the start or eachrun and were reweighed, after polishing; at the end ortherun.

The losses in weight were calculated as milligrams per hour per square inch oi" exposed surface. At

the end of this run, the solution contained a large hada decided aldehyde In addition, carbon had deposited on the walls of the chamber. The corrosion rates of the test specimens were: aluminum, 0.01 iron, 0.00; brass,

Ina comparativetest in accordance with the system contained 2320 cc. of a mixture of 70 parts of ethylene glycol, 30 parts of water, and 0.5 partof 1,4-piperazinediethanol,

the initial pH of which was 8.7, This mixture was also circulated for 100 hours with the chain her at 200 C. At the end of this. period, the

solution was entirely clear and free of particles,

had no aldehyde odor, and was still basic, the was no formation of carbon or tar in the chamber. The corrosion rates of aluminum, 0.01 iron,

0.00; brass, 0.02; and copper, 0.01. v

. Example 2. Undiluted lycol at 300 C.

QIn a blank run," Y 1020 cc, of undiluted ethylene glycol containing no inhibitor, the pH of the glycol being 7.0. The chamber was maintained at about 300 C., circulation being continued for 12.5 hours, atwhich timeit had to .be stopped because of excessive deposition-of carbon and tar in the heated chamber. The glycol was dark brown in color, had a strong aldehyde odor, and was acidic, thev pH being-5.8. The metal test specimens were so coated 'withcarbon that corrosion rates could In a; comparative test according to the invention, the system contained 2250 cc. of undiluted ethylene glycol containing 0.5 per cent by Weight of 1,4-piperazinediethancl, the pH of the mixture being 8.2. 'Thls liquid was" circulated for 100 hours with the chamber at about 300 C.v At the end of this "intervalthe solution was entirely clear, and had no aldehyde odor, but had turned a light reddish color; the pH was 7.8. No carbon had formed, and there were onlyslight traces of a light tar inthe chamber. The metal corrosion rates were: aluminum, 0.04; iron, 0.09; brass,

0.05; and copper 0.00.]

It will be appreciated that both Examples l and also ,to observethe rates of.-

the system was charged with a, stabilized. against plpera'z'inediethanol in a proportion of from 0.2'

. .metalspecimens,

thermal decomposition by 1.4-

to 5 percent by "weight of the 317001.

2. A heat-vtransfer medium chemically stable at elevatedt'emperatures consisting essentially of ethylene glycol and from about 0.2 to about 5 p'er cent by weight thereof of 1,4-piperazinedlprolonged period at :tiallyot one part y I over one part oi.- water,

. 1,4-piperazinediethanol; v

=; 4. In-a.1process' wherein a Heat-transfer meglycol containing H perv molecule and not ethanol.

- 3. A liquid heat-transfer chemically stableatselevated, temperatures consisting essenweightot ethylene glycol, not

and.0.002 to 0.05 part of essentially of a lower alkylene from2 through 5 carbon atoms over ancqual proportion by weight of water is heated as a liquid {or a an elevated temperature,

dium consisting the method of l in a proportion oi'vfrom 0.2 to 5' per theglycol. r

heated as a liquid at an elevated temperature of at least 200". 0., the method'of minimizing decomweight of 1,4-piperazinediethanoL- 2oo:c., the method'ot "I. In a process wherein a liquid mixture of ethylene glycol and a lesser proportion of water elevated temperature oi at least decomposition solved therein 1,41-piperazinediethanol in a proglycol.

file of this patent:

EDGAR c. nnrrron. ARTHUR R. snx'ron. .anrnanncns crrnn c The iollowingreferences are of record in the UNITED STATES PATENTS j Number lJate r 1,903,287 Cox 1 Apnei. 1933 1,988,584 Dana et al Jan. 22, 1935 2,060,138 Taylor Nov. 10,1936 2,078,256 Liebe-r etal Apr. 27, 1937 2,164,564v Britten et al. 1 July-i, 1939 weight oi" from 0.2 to5 per cent by -weight ot the 

